Hello BOAC- you are quite right, a tip stall on a swept wing would move the centre of pressure forward........but.......because a tip stall is such a dangerous phenomenon (especially if it is asymmetric- maybe due to sideslip/gust), it is designed out in a large transport. Hence tips tend to wash out and reduce angle of attack, leading edge profiles change near the tip, and/or sweepback reduces (most visible I think on the Victor). Look at wingtips of most large jets, you can usually see several features designed to ensure wingtips don't stall first !